Pull up revision 1.101 (requested by thorpej in ticket #474):
Fix some locking issues:
- Make the locking rules for pr_rmpage() sane, and don't modify fields
protected by the pool lock without actually holding it.
- Always defer freeing the pool page to the back-end allocator, to avoid
invoking the pool_allocator with the pool locked (which would violate
the pool_allocator -> pool locking order).
- Fix pool_reclaim() to not violate the pool_cache -> pool locking order
by using a trylock.
Reviewed by Chuq Silvers.